1. Field of the Invention
This invention relates to the provision of coated abrasives, particularly those suitable for precise surface texturizing, fine finishing and/or burnishing of surfaces to precise tolerances with respect to both smoothness and curvature. Coated abrasives generally comprise a flexible web backing and a layer of abrasive grain which is attached by one or more layers of adhesive to at least one of the major surfaces of the backing. The products provided by this invention are most particularly suited to finishing the read-write heads used in conjunction with recording discs or tapes, laser components of optical character readers and pattern scanners, and electromagnetic recording media such as the floppy and hard discs used in computers. This invention also relates to the field of radiation curable adhesives.
2. Technical Background
In the prior art, most coated abrasives have been made with adhesives of animal glue or of synthetic resins, usually thermosetting resins such as urea-formaldehyde or phenol-formaldehyde. The vast majority of prior art coated abrasives have also been backed with fabrics or with cellulosic non-fabric webs such as paper or vulcanized fiber. Such products are suited to known processes of curl correction which involve control of the moisture content of the backing combined with flexing, a mechanical process which forces the coated abrasive web to pass over one or more surfaces with a small radius of curvature. Conventional flexing processes are performed at ambient temperature and usually without any source of mechanical pressure on the side of the web bearing abrasive grain. Conventional flexing breaks the abrasive coating up into small "islands" and allows the curvature of the entire coated abrasive product to be controlled essentially by the curvature of the backing alone.
These known curvature control processes are satisfactory for the types of coated abrasives already noted, but they are not generally satisfactory for coated abrasives with solid plastic films backings and adhesives suitable for rapid curing induced by radiation. Solid plastic film backings are preferred for applications requiring precise dimensional control of the finished objects, because the plastic can be economically obtained with very smooth and flat surfaces. Adhesives suitable for rapid curing induced by radiation are preferred for economy and speed of manufacture, as further explained at lines 7-23 of page 1 of application Ser. No. 680,619 entitled "Improved Coated Abrasive with Rapidly Curable Adhesives" and filed Dec. 7, 1984 now abandoned. This application is briefly cited hereinafter as Supkis, the first inventor listed on the application.
The only published instance of a coated abrasive prepared by UV curing known to us is in Japanese Laid-Open Application No. 119491/1978, dated Oct. 18, 1978. This document indicated that the presence of an isocyanate compound in the adhesive is important for success with UV light initiated cure of adhesives for coated abrasives. Furthermore, although it was generally asserted in this Japanese publication that all the formulations disclosed therein are suitable for cure by UV light as well as electron beam curing, only one of the sixteen specific examples actually used UV light.
A waterproof paper coated abrasive with fast curing adhesives was disclosed in U.S. Pat. No. 4,047,903 to Hesse et al., but this product was cured by electron beam radiation only.
The above cited Supkis application describes general purpose coated abrasives with radiation curable adhesives and also certain specific embodiments adapted to lens fining operations. It has been found that these embodiments are often insufficiently flat as produced to be satisfactory for finishing of objects which require very high flatness, or other precisely controlled curvature, over their surfaces in order to operate satisfactorily. In order to achieve precise curvature on a workpiece, the coated abrasive used to finish it must be capable of approximately equally precise curvature control. Such control over the curvature of the plastic film backed products described in the Supkis application has not been achieved with normal curl correction processes used for conventional coated abrasives. (It is suspected, although not investigated by us, that the initial lack of flatness of these Supkis products is connected with the tendency of radiation curable adhesives to shrink upon cure.) It is an object of the present invention to provide products with radiation curable adhesives susceptible to precise curvature control.
The general problem of controlling the shrinkage of thermosetting resins upon cure by addition of thermoplastic resins has been treated by numerous workers. One recent example is Kwok-Wai Lem and Chang Dae Han, "Chemorheology of Thermosetting Resins. III. Effect of Low Profile Additive on the Chemorheology and Curing Kinetics of Unsaturated Polyester Resins", 28 Journal of Applied Polymer Science 3207-3225 (1983). This article notes on page 3207 that "the mechanism(s) by which low-profile additives reduce shrinkage in . . . thermosetting . . . compounds is still not clearly understood today." In view of this and other literature known to the applicants, little or nothing about shrinkage prevention in one type of base resin can be inferred from specific data obtained with another type.
U.S. Pat. No. 4,407,855 to Russell describes the preparation of abrasion resistant, radiation curable coatings from cellulose esters and pentaerythritol acrylates. A weight ratio of acrylate to cellulose esters between six and one hundred is specified. No suggestion of any application of the adhesive compositions disclosed in the Russell patent to coated abrasives is made in the specification of the patent.
The use of dopants in addition to mercury to increase the amount of light in the wavelength range between 366 and 420 nanometers (nm) in the emission spectrum of ultraviolet (hereinafter UV) lamps is described in S. Peter Pappas (editor), UV CURING: Science and Technoloqy, Vol. II, p. 64-66 (1985). An example of the use of such lamps to cure pigmented radiation curable adhesives, in this case a mixture of acrylated epoxy resin and n-butyl acrylate pigmented with titania, is A. Aldridge, P. Francis, and J. Hutchison, "UV Curing of TiO.sub.2 Pigmented Coatings; Evaluation of Lamps with Differing Spectral Characteristics", 11(3) Journal of Radiation Curing 10 (July 1984).